Nanoscalar electronic components, such as nanowires, have tremendous potential to accurately measure the presence and amount of molecules in solution. In particular, devices utilizing such nanoscalar components are useful to detect very low concentrations and require only small volumes in the order of a few picoliter.
The basic measurement principle is to apply a surface treatment to the nanoscalar component to which the molecule of interest can be adsorbed, based on a chemical binding preference of the molecule to the surface treatment. Upon binding of the molecule to the surface treatment, an electrical property of the nanoscalar component changes as a result of the binding. The binding preference can be the electrostatic attraction of oppositely charged ions, hydrogen bonds formed between complementary strands of DNA, or an antibody binding to an antigen. For example, the conductance of the nanoscalar channel of a field effect sensor can be modified significantly, when biased under subthreshold conditions, upon the binding of the molecule to the surface treatment and a measurement of the change in conductance allows detecting the presence of the molecule. Upon calibration with solutions containing the molecule at a known concentration, an unknown concentration can be determined from the calibration data.
Biosensors having nanoscalar components on which a surface treatment is applied are known. The sensitivity of the nanoscalar components increases with decreasing thickness of the components. Further, a decreased thickness allows the incorporation into devices for small sample volumes. However, it has been found that the thickness of the nanoscalar component may not be decreased beyond a certain point because processing conditions, such as the implantation of impurities can cause decrystallization of the semiconductor material of the nanoscalar component resulting in amorphous regions. Such amorphous material is no longer suitable for the intended measurement because the resistivity increases significantly. For nanoscalar components with a thickness of several tenths of a nanometer, recrystallization with accompanying decrease in resistivity can be satisfactorily obtained by an annealing step. However, for ultra-thin nanoscalar components, i.e. for thicknesses below 10 nanometer, recrystallization cannot be achieved satisfactorily. Thus, low resistance channels cannot be formed.
Schottky source/drain (S/D) structures are an alternative because decrystallized silicon can be replaced by much more conductive metal silicides. However, ordinarily Schottky S/D structures on the above-described thicknesses of 10 nanometer or less suffer from misaligned workfunctions resulting in degraded subthreshold slope, which determines the device sensitivity. The sensitivities of devices with a degrades subthreshold slope are insufficient for the low detection levels envisioned herein.